Benzo-l,3-dithiol-2-thione

Explicitly included are species with carbonyl (>CO) groups such as the isomeric phthalimide (VIII) and isatin (IX), and with thiocarbonyl (>CS) groups such as the isomeric benzo-l,2-dithol-3-thione (X) and benzo-l,3-dithiol-2-thione (phenylene trithiocarbonate) (XI). Conversely, species with the isoelectronic >BF (and the related >BH) are ignored. Thus the question of the aromaticity in carboranes never arises in this chapter, even had we been explicitly interested in three-dimensional aromaticity (another issue we will ignore here). 

Finally, we turn to sulfur-containing species. Many indane derivatives may be suggested. However, there are only a few for which the enthalpy of formation has been determined phenylene trithiocarbonate (benzo-l,3-dithiole-2-thione) (X = Z = S, Y = CS) (XI), the isomeric benzo-l,2-dithiole-3-thione (X = Y = S, Z = CS) (X) [50],... 

Benzo-l,3-dithiole-2-thiones (254) are cleaved to thiocatechols (256) by treatment with DIBAL-H, or diborane-dimethyl sulfide. Sodium borohydride, LAH or Red-Al are much less efficient reagents for this purpose. Under more controlled conditions it is possible to obtain good yields of the intermediate ben-zodithiolanes (255). ... 

Reduction of l,3-dithiole-2-thione yields an anion-radical (150) whose ESR spectrum at low temperature has been measured the very low proton splitting (indicated in gauss) implies the spin population to be highly concentrated in the trithiocarbonate moiety. Consistent with this, no proton hyperfine splittings were resolved in the ESR spectrum of the anion-radical of benzo-l,3-dithiole-2-thione. ... 

Thermolysis of 1,2,3-thiadiazoles (311) in carbon disulfide under forced conditions provides a synthetic route to trimethylene- and tetramethylene-l,3-dithiole-2-thiones and benzo-l,3-dithiole-2-thiones (233) (Equation (63)) <61E566, 76JOC730). Alternatively, a photochemical path has been devised <61E566>. 

A detailed discussion has appeared of reversible redox systems based on the bi-dithiole (74 R = H) and the related cyclohexeno- and benzo-com-pounds, the last of which is best prepared by the action of ethyl phosphite on benzo-l,3-dithiole-2-thione. 

The 8 + 2-cycloaddition of 2//-cy c I o h cpta[h ] furan -2 - one (155) with acyclic 1,3-dienes provides a facile route to bicyclo[5.3.0] ring systems (156) (Scheme 61).291 2H-Benzo[Z>]thiete in the o-quinoid form undergoes 8 + 2-cycloaddition with 1,3-dithiolane-2-thione, l,3-dithiole-2-thiones, and adamantanethione to produce 4//-1,3-benzoditliianes.292... 

Benzo[7]thiete 41 reacts in the ortho-qumoiA isomer 41 with cyclic trithiocarbonates such as l,3-dithiolane-2-thione, (ethylene trithiocarbonate), l,3-dithiole-2-thiones, and adamantanethione <1997LA1603>. In boiling toluene, 1,3 dithiolane and dithiole derivatives formed the spiro compound 42 and 43 in good yield. The related l,3-dithiole-2-thiones reacted chemoselectively at the C=S double bond. The use of adamantanethione yielded the polycyclic adduct 44 in a smooth and quantitative reaction (Scheme 3). 

Quantum mechanical calculations for the 1,3-dithiolylium ion, benzo-1,3-dithiolylium ion, l,3-dithiol-2-one and l,3-dithiole-2-thione have been carried out with the simple LCAO MO method <66AHC(7)39). According to these calculations, the lowest electron density was found at the 2-position of the 1,3-dithiolylium cation, and the C(4)—C(5) bond order corresponded approximately to that of an isolated C—C double bond. A delocalization energy of 105 kJ mol-1 has been calculated for this cation. For the benzo-l,3-dithiolylium ion it was found that nucleophilic attack should take place in the 2-position but electrophilic substitution at the 4-position. 

This section is divided into reactions of fully unsaturated rings like 1,3-dithiolylium ions, mesoionic 1,3-dithiolones, l,3-dithiol-2-ones and l,3-dithiole-2-thiones, and reactions of the saturated 1,3-dithiolanes. As a consequence of the positive charge in 1,3-dithiolylium ions, the main reactions of this class of compounds consist of nucleophilic attack at the 2-position, whereas the mesoionic 1,3-dithiolones undergo cycloaddition reactions. Reactivity of the benzo ring in benzo-l,3-dithiole and related systems has not been studied to any extent. 

An understanding of certain chemical and physical properties of sulfur-containing heterocycles has been offered by quantum chemical calculations. Some earlier computational results by the Hdckel LCAO-MO method have already been summarized <65AHC(5)i). In addition, ir-electron densities and reactivity indices of 4-aryl-1,3-dithiolylium ions have been calculated <65ZC23, 70CPB865>. Simple LCAO-MO calculations for the l,3-dithiole-2-thione, benzo-1,3-dithiolylium ion,... 

Spectroscopic data for l,3-dithiole-2-thiones and l,3-dithiole-2-ones and their corresponding saturated compounds have been compiled <81JPR737>. Calculated electron densities have been correlated with observed C chemical shifts for the benzo-1,3-dithiolylium ion <77CLI133>. 

Benzo-anellated l,3-dithiole-2-thiones (254) are also obtained by the reaction of the dithiols (253) with thiocarbonyldiimidazole in glacial acetic acid or with carbon disulfide in alkaline solution (Equation (42)) <76JOC145, 83JOC4713, 83MI 312-05,85MCLC295>. 

There are no reports of aromatic substitution reactions on benzo- or dibenzotetrathiafulvalenes. With regard to the reactivity of the substituents on the benzene ring(s), the main examples involve 1,3-dithiol-2-ones and l,3-dithiole-2-thiones. The former afford octakis(alkylthio)dibenzote-trathiafulvalenes 25 through basic ring opening and subsequent alkylation... 

Reduction of l,3-dithiole-2-thione and its benzo-derivative with lithium aluminium hydride gives ethanedithiol and thiocatechol respectively, but similar reduction of the cyclohexene compound (59) replaces the thio-carbonyl group by a methylene group. ... 

Benzo-l,3-thioxin-4-thione (148) reacts with phenylhydrazine to form the dithiole hydrazone (149), which must arise by initial nucleophilic attack at the thione function (80T3309). The l,3-thiazine-2,4-dione (150) possesses a three-carbon unit with an attached sulfur atom. Thionation with phosphorus pentasulfide gives the unsubstituted l,2-dithio e-3-thione (3b) (70AjC5l>. 

Benzo-l,2-dithiole-3-thione (77b) also reacts, but the monoadducts corresponding to (95) have o-thioquinone methide structures (cf. structure 104) and form diadducts rapidly. The use of dehydrobenzene (benzyne) gives 2-thioacylmethylenebenzo-l,3-dithioles (95 R3, R4 = (CH=CH)2). 

The parent 1,2-dithiolylium ion (4) is readily prepared by treatment of l,2-dithiole-3-thione (3b R = R = H) with hydrogen peroxide in acetic acid (65JCS32). The method may be applied to the alkyl and aryl derivatives with equal success. For cations with 3- and 5-substitution the acid catalyzed reactions of j8-dicarbonyl compounds with hydrogen disulfide or equivalent are best (80AHC(27)l5i), whereas the benzo-1,2-dithiolylium ion (172) and related compounds are best prepared by ring contraction of benzo-l,3-dithiins (171) (63LA(661)84>. 

Thione-l,3-dithiole-benzo [Pg.469]

Synthesis of fused D-A systems such as 665 involved /-butyldiphenylsilyl protection of 4,7-dihydroxy-l,3-benzo-dithiole-2-thione 661, then the Horner-Wittig reaction of the resulting 662 with the carbanion derived from phosphonate 663 and finally deprotection of the TTF 664 to the required 665, obtained as green crystals (Scheme 94) <2003AG2871, 2004JOC2164>. 

Some years later, the first stable thiocarbonyl ylides 9 and 10 were prepared by the reaction of thiourea with cyano-substituted oxiranes (19,20) or by addition of Rh-di(tosyl)carbenoid to benzo-l,2-dithiole-3-thione (21), respectively. Enhanced stability and the low reactivity of 9 and 10, which enables their isolation in crystalline form, results from the push-pull substitution at the two termini [cf. also (22)]. Another class of stable thiocarbonyl ylides that are also able to afford [3 + 2]-cycloaddition products are the mesoionic 1,3-dithiole-4-ones of type 11 (23,24). 

Benzo-l,2-dithiole-3-thione+2C2H6 — oC6H4(CH3)2 + CH3C(S)SSCH3 (21)... 

The photolysis of 4-phenyl-l,2-dithiole-3-thione (71) gives a dimer (72) (73TL1561), but benzo-l,2-dithiolane 2,2-dioxide (73a) gives an o-thioquinone methide (74) (78JOC3374). 

The reduction of benzo-l,2-dithiole-3-thiones (77b) to various polythiols (74ACS(B)827, 63LA(661)84) may be accomplished by lithium (78BEP867155), lithium aluminum hydride, or by catalytic hydrogenation, and Raney nickel hydrodesulfurization gives products by complete elimination of sulfur (B-66MI43100). 

For alkanes, alkenes and their aryl derivatives, direct sulfurization gives thiones (3b). Typical examples are the preparation of 4-methyl- (10), 5-(4-methoxyphenyl)- (129) and 4-phenyl-l,2-dithiole-3-thiones (71) and benzo-l,2-dithiole-3-thione (77b) (Scheme 22) (B-66MI43100). 

The readily accessible enamines react with carbon disulfide and sulfur under mild conditions to produce l,2-dithiole-3-thiones (3b) via 3-amino dithioacids (Scheme 24), The nucleophilic character of the enamines is necessary for the initial reaction and to activate the sulfur (from Sg) for further insertion (67AG(E)294). Modified procedures produce other heterocycles. The yields of thiones (3b) may be low but this versatile reaction produces thiones that may be otherwise difficultly accessible, especially by direct sulfurization procedures, e.g. 1-morpholinocyclohexene may be converted into tetrahydrobenzo-1,2-dithiole-3-thione (142) in 40% yield. By contrast, direct sulfurization of 1-methylcyc-lohexene gives benzo-l,2-dithiole-3-thione (77b). Even dihydronaphtho-l,2-dithiole-3-thione (143) may be made by this procedure, although this compound may be dehydrogenated readily by sulfur at 220 °C. 

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